The influence of lithology on the Magnitude–Frequency-Distribution of earthquakes

The earthquake Magnitude-Frequency-Distribution, FMD, is usually modelled with the Gutenberg-Richter relation law, where the b-value controls the relative rate of small and large earthquakes. b-value has been documented to show an inverse dependence on differential stress, it increases with the fault roughness or during fluid-induced earthquakes. For some seismic sequences a near real-time characterization of the b-value has been used to discriminate between foreshocks and aftershocks. Here we examine the influence on b-values of different lithologies hosting earthquakes.

In general, seismicity not only localizes along the major structures where mainshocks nucleate, but it can be also distributed within volumes of the seismogenic layer characterized by different lithologies. For the Mw 6.5 2016–2017 Central Italy seismic sequence, the lithology can be properly defined by seismic reflection profiles. Here the fractured carbonate of the Apennines, located at almost 1-2 km and 4-6 km of depth, are characterized by b-values ranging between 1.3 and 1.4 that can be diagnostic of brittle dominated deformation. At 2-4 km and 6-10 km of depth, the Triassic Evaporites showing a bimodal brittle-ductile deformation and compartmentalized fluid overpressure (documented in deep boreholes) are linked to high b-values, in the range of 1.5-1.65 reaching 1.80 for clustered swarms. Between 10-12 km of depth the phyllosilicate rich basement, with its predominant velocity strengthening behaviour, is hosting small magnitude earthquakes with b-values around 1.4. Our results indicate that away from the large earthquake faults, characterized by a stress dependent elasto-frictional rheology, FMD are strongly controlled by rock lithology and style of deformation.